1. Field of the Invention
The present invention generally relates to light-emitting semiconductor devices and, more particularly, to superluminescent diodes.
2. Description of the Prior Art
Numerous semiconductor devices capable of light-emission have become known in recent years. Among these devices are the light-emitting diode (LED) and the semiconductor laser. LED's function by injecting holes and electrons into an active region at a junction of the diode. Light emission is then caused by the spontaneous recombination of electrons and holes. This radiative recombination is made to predominate over other equilibrium recombinations by the development of a potential barrier (i.e. band-gap, as distinct from a potential barrier of a p-n junction) which limits a majority of electron energies at the junction to levels at which radiative emission will occur.
Since the recombination is spontaneous and all electrons and holes must be injected as a current applied to the LED, efficiency is low. Spectral purity is also low due to the plurality of energy states at which recombination may take place.
In contrast, semiconductor lasers have much higher efficiency due to gain attributable to photons emitted from a recombination of an electron-hole pair which stimulate radiative recombination of other electron-hole pairs and thus stimulate emission of radiation in a common wavefront of uniform phase, thereby creating a coherent beam of light. These effects are enhanced by containment of light within a resonant cavity.
The light produced by a semiconductor laser is not only coherent but also of high spectral purity. Because of these qualities of light and the high efficiency typical of lasers due to the stimulated recombination of electron-hole pairs, engendering gain, many applications have been developed in which lasers can be advantageously used. Since the light output from lasers can be modulated at very high frequencies, lasers have been applied to communication systems, often in combination with fiber optics.
Lasers have also been used in combination with fiber optics in so-called fiber optic gyroscopes which have recently been widely used in high accuracy guidance and navigation systems. However, some difficulties have been encountered by noise generated by Rayleigh backscattering in the fiber, which causes optical feedback to the laser. This feedback affects the laser gain and, therefore, the amplitude of light output of the laser, engendering noise and reducing the accuracy of the gyroscope. Laser performance under conditions of external optical feedback is also complicated by Fabry-Perot modulation of the light wavelength. Fabry-Perot modulation is due to alteration of the index of refraction within the laser and, therefore, the effective length of the resonant cavity, with changes in light intensity.
Similar problems occur but with even greater severity in fiber-optic communication systems because the length of the fiber-optic light guides is far greater and the light path will also include interconnection and coupling surfaces which are inherently partially reflective. In fact, feedback of light to the laser from reflections in the fiber-optic system often predominates over Rayleigh backscattering as a cause of noise in the system.
Fabry-Perot modulation can be reduced by reduction of light intensity within the resonant cavity of the laser. Since the laser inherently operates by developing high light intensity in a resonant cavity, however, reduction of light intensity in the resonant cavity is inconsistent with laser operation. Nevertheless, stimulated emission of light with gain can occur at lower light intensity and reduction in the coherence length of the emitted light. This can be done in several ways already known in the art.
A first method of reducing coherence length and broadening of the spectral bandwidth of the laser is to tilt the waveguide or resonant cavity of known double-heterostructure lasers at a slight angle to reduce the internal reflectivity of the resonant cavity. A second method is to rely on coatings at the ends of the resonant cavity to decrease reflectivity as in Burnham et al., U.S. Pat. No. 4,730,331 or to provide a light absorption portion of the waveguide as disclosed in Noguchi et al., EPO 0 318 947 or Kwong et al., U.S. Pat. No. 4,764,934. Both of these approaches allow virtually all the light within the resonant cavity to escape and thus reduces the stimulation of electron-hole radiative recombination by the reflected photons. These approaches result in reduction of the coherence length of the emitted light. Providing for escape of a sufficient amount of light to prevent the lasing action is not trivial since gain will occur due to a single pass of photons through the body of the material (i.e. the cavity) and containment of only about 1% of the light produced is necessary to support a lasing action. The reduction of internally reflected light reduces light intensity within the cavity and greatly reduces Fabry-Perot modulation. Some of the device gain is lost but efficiency is still far higher than that of LED's since stimulated recombination continues to occur with achievement of net gain during the single pass of photons through the material.
All of these methods of broadening the spectral output effectively reduce the coherence length of the emitted light. In essence, these effects cause the laser to operate more like an LED (that is, for the LED, no Fabry-Perot modulation, broad spectrum, very-low intensity spontaneously emitted, non-coherent light output) by reducing optical feedback amplification while still providing a much increased efficiency and light output capacity. Net gain is maintained in these devices through stimulated emission (i.e. stimulated radiative recombination of electron-hole pairs). Substantial efficiency may be maintained even when the resonant modes of the cavity are entirely suppressed since gain remains available from the single pass stimulated recombination of electrons and holes. Such laser-like structures operating at short coherence lengths are generally referred to as superluminescent or superradiance diodes. A more detailed description of such devices, and a solution to the problem of noise due to Rayleigh backscattering can be found in High-power Low-divergence Superradiance Diode by Wang et al.; Applied Physics Letters 41(7), October, 1982; pp.587-589 and High-Power Superluminescent Diodes by Alphonse et al.; IEEE Journal of Quantum Electronics, Vol. 24, No. 12: December, 1988; pp. 2454-2457, both of which are hereby incorporated by reference.
To date, while many different structures have been used in semiconductor lasers, the most widely used electronic structure is known as a double-heterojunction. (While the term double-heterojunction is more properly used to refer to the electronic properties of a semiconductor structure known by the term double-heterostructure (which, properly used, refers to the metallurgical structure), these terms are sometimes used interchangeably.) Virtually all superluminescent diodes based upon laser structures are modifications of the double-heterojunction laser configuration described in the above incorporated publications. Waveguides of semiconductor lasers fall into two types; gain guided, where the cavity boundary is formed at the boundary of the pumped region (causing the geometry of such configuration to vary significantly with temperature) and index guided, using a change in index of refraction to form the boundary of the cavity or waveguide to contain the light at less than a critical angle to the axis of the cavity.
Examples of fabrication of SLDs from these types of double-heterojunction laser are taught in U.S. Pat. No. 4,789,881 to Alphonse, which relies on an external cavity for operation; U.S. Pat. No. 4,821,276 to Alphonse et al., which relies on crystal cleaving or etching to achieve the equivalent of an inclined stripe, used to obtain off-axis reflection; U.S. Pat. No. 4,821,277 to Alphonse et al., which includes a gain-guided waveguide; and U.S. Pat. No. 4,856,014 to Figueroa et al. (including the present inventor) which uses an index-guided waveguide structure, all of which are also hereby incorporated by reference.
For reasons of convenience in the manufacture of superluminescent diodes (SLD), it has been the practice in SLD design to alter double-heterojunction semiconductor lasers by modifying the laser stripe defining the waveguide orientation so that the cavity is inclined at a slight angle to a direction perpendicular to the facet mirrors of at least one-half of the critical angle above which total internal reflection does not occur in the channel waveguide or about 5.degree., in most cases, but possibly as low as 3.degree.. This condition will also be met whenever there is no line perpendicular to an end facet and intersecting that end facet that intersects the opposite end facet. Additional design criteria for the method of inclination of the waveguide to the facets is described in Alphonse et al, U.S. Pat. No. 4,821,277, and Figueroa et al, U.S. Pat. No. 4,856,014, both incorporated by reference above. However, several problems occur when this or other approaches to forming SLDs from laser structures are done.
Known SLDs designed in any manner characteristically have very high threshold currents, generally two to three times the threshold current required to produce laser operation in comparable laser designs. The increase in threshold current is due almost entirely to the lack of positive feedback of the photon containment. This level of threshold current requires complicated cooling arrangements and power supplies for operation of the SLD in comparison with the power supplies and cooling arrangements required for lasers of comparable design. This threshold current is also very temperature sensitive, complicating necessary drive circuitry and reducing reliability and useful lifetime of the superluminescent diode. The temperature sensitivity also complicates the design and operation of cooling arrangements if consistent operation is to be obtained. The differential quantum efficiency is also generally reduced in comparison with similar laser designs.
Another electronic structure used in semiconductor lasers is the quantum-well heterojunction. This configuration differs from the double-heterojunction laser configuration principally in the thickness of the active region bounded by the two highly doped confining layers typical of the double-heterojunction. When this active layer is reduced to the order of the de Broglie wavelength, effects are encountered which are not typical of the bulk material. A detailed discussion of the structure, method of manufacture and theory of operation of quantum well lasers is provided in Quantum-Well Heterostructure Lasers, by Nick Holonyak, Jr.; IEEE Journal of Quantum Electronics, Vol.QE-16, No. 2: February, 1980; pp. 170-186, which is also fully incorporated by reference. A quantum well semiconductor laser is also disclosed in Hayakawa et al., U.S. Pat. No. 4,841,533. In essence, the quantum well is formed by the thinness of the active region which causes the density of states of electrons in the active region to become quantized and confined to a finite potential well. The usual band-to-band recombination process is therefore modified in a fundamental manner.
As taught at page 171 of the Holonyak article, in a double-heterojunction where the density of states is not quantized, the electrons and holes can recombine at any of a continuum of energies and the density of states varies parabolically with energy. Therefore, electrons and holes cannot all be of fixed energies or recombine in a narrow line width or wavelength. However, in a quantum well laser, electrons thermalize by photon generation to quantized states and the electrons confined in the quantum well are, in principle, of a single, fixed energy state. These confined electrons are confined with holes which are also at a fixed energy which enhances recombination with electrons. Moreover, the thermalization of electrons is not limited by the decreasing density of states characteristic of the double-heterojunction lasers and thermalization is more efficient, allowing electrons and holes to be moved into the active region at energies below that of the confined particles. This yields a significantly lower threshold for laser operation in comparison with the double-heterostructure configuration. In addition, temperature sensitivity of threshold current is somewhat reduced, relative to double-heterojunction lasers.
While the quantum well configuration for semiconductor lasers has been known for a number of years, the formation of SLD's based on laser designs is a relatively recent development and, as indicated above, has been confined to modification of double-heterojunction lasers. The feasibility of forming SLD's based on quantum well laser designs or otherwise providing quantum well structures in SLD devices has not, heretofore, been investigated.